Though he finds Light interesting, Ryuk doesn't truly care for his well being as he doesn't care if he lives or dies and does not actively assist him. His only purpose in giving him the Death Note was because he was bored and most of his actions are primarily motivated by wanting to see something interesting, as he sees Light's use of the Death Note and the way humans react to the Death Note as truly interesting. He is different from most shinigami in that he does not want to simply wither his life away in the shinigami world doing pointless games but to be actively involved in the human world to detain his boredom, something other shinigami mock him for.

Otaku Trivia:

1) In every adaptation, Ryuk's wings are some manner of evil. In the manga, he has black crow wings, while in the film adaptation he has leathery bat wings.

2) Apples are addictive to Ryuk.

3) In the TV drama, Ryuk intentionally drops the Death Note near Light, and goads him into using it more when Light hesitates, effectively making him the one responsible for Light becoming Kira, a stark contrast to the source material in which he was a self-acknowledged neutral bystander.